Sleep apnea is a common disease with broad health and economic impact. For most adults with sleep apnea, continuous positive airway pressure (CPAP) is first-line therapy. CPAP has many beneficial effects on acute disease burden (e.g., daytime somnolence, quality of life decrements, performance deficits) and on associated cardiovascular risk, but these benefits are clearly tied to patient use of the therapy. Many variables are thus used to predict CPAP use, but their value is inconsistent, even for those commonly used, such as the apnea-hypopnea index (AMI). This project aims to address this problem by evaluating the predictive value of the comprehensive Sleep Apnea Severity Index for patient CPAP use. This validated index combines several measures of disease burden into a single composite measure. Prediction of CPAP clinical outcomes is similarly unclear. No current single measure assesses effects of therapy on the diverse aspects of disease burden experienced by sleep apnea patients. So, this project will also evaluate the Sleep Apnea Severity Index as a predictor of the various treatment effects. This is a prospective cohort study of newly diagnosed sleep apnea patients followed over six months of CPAP therapy. Data collected will include questionnaires, physical examination, sleep testing, and CPAP use. CPAP use and outcome measures will be compared between strata of predictor variables in order to test their relative predictive value. Relevance: This study is the first to test a comprehensive index as a predictor of use and effects of CPAP, the first-line therapy for adult sleep apnea. The Sleep Apnea Severity Index combines currently used predictors of CPAP use and outcome, so it may be a new and useful single tool for clinicians caring for sleep apnea patients and for scientists investigating the disease and its treatments. As such, it may also have broader implications for evaluation and management of major sleep apnea-associated health problems in the United States, such as heart disease, stroke, and traffic accidents, as well as for billions of dollars in sleep apnea-related healthcare costs. [unreadable] [unreadable] [unreadable] [unreadable]